The high affinity IgE receptor (FcepsilonRI) belongs to a class of receptor which lacks intrinsic enzymatic activity but activates non- receptor tyrosine and serine/threonine kinases and phosphatases. This activation leads to the phosphorylation and activation of phophatidylinositol-specific phospholipase C, an increase in intracellular calcium and ultimately to cell degranulation and the release of the mediators of allergic reactions. We have previously established that the beta chain of FcepsilonRI binds the tyrosine lyn and that the gamma chain of FcepsilonRI activated the tyrosine kinase syk. Using various recombinant vaccinia viruses engeneered in the laboratory, we have no reconstituted, in a null fibroblastic cell line, a minimal signaling complex that includes the alpha beta gamma2 FcepsilonRI and the tyrosine kinases lyn and syk. In this complex like in a mast cell spontaneously expressing FcepsilonRI, clustering induces receptor tyrosine phosphorylation and syk activation. Upon triggering, lyn phosphorylates the beta and gamma chains and phosphorylates and activates syk. These events do not require a heamatopoietic-specific phosphatase. However, an unidentified phosphatase present in this fibroblastic cell line is probably responsible for preventing triggering-independent phosphorylation of the receptor. Triggering through this minimal complex does not result in the phosphorylation of substrates downstream of syk similar to the ones that are phosphorylated after triggering of a mast cell. We are currently exploring the possibility that other elements are part of the signaling complex of FcepsilonRI and could be responsible for the involvement of downstream elements resulting in degranulation and the release of allergy mediators.